In order to observe the morphology of a sample, such as a biological tissue, and simultaneously measure the distribution of the molecules existing in a specified area on the sample, apparatuses called a mass microscope or an imaging mass spectrometer have been developed (refer to Patent Documents 1 and 2, Non-Patent Documents 1 and 2, and other documents). These apparatuses require no grinding or crushing of the sample (as required in conventional apparatuses) and hence are capable of mapping molecules included in any area on the sample specified based on a microscopic observation while almost completely maintaining the original morphology of the sample. These apparatuses are expected to be used, for example, to obtain distribution information on the proteins included in a living cell, particularly in the fields of biochemistry, medical care, pharmaceutical chemistry, and other fields.
The ionization method mainly used in such a mass microscope is a matrix-assisted laser desorption ionization (MALDI) method or a secondary ion mass spectrometry (SIMS) method. In some cases, a desorption electrospray ionization (DESI) method is alternatively used. In any methods, in analyzing a sample such as a biological tissue, the sample is sliced and attached to a sample plate. Then, a pretreatment such as a matrix coating is performed according to necessity and the sample is set to the apparatus. A small focused laser beam or ion beam is delivered onto the sample and the irradiation position is changed thereon, thereby performing a mass analysis for a specified area on the sample to obtain, for example, a mass analysis result image for a specific mass-to-charge ratio.
With the previously described conventional measurement methods, a mass analysis result image can be obtained for a specific mass-to-charge ratio in a specified two-dimensional area on a sample. However, information of the mass analysis in the depth direction of the sample cannot be obtained. In some studies, an attempt has been made in which a plurality of sample slices are prepared by sequentially slicing one biological sample and the sample slices are sequentially analyzed to obtain three-dimensional mass analysis information.
However, such a method requires an extensive amount of time and labor; therefore, it is impractical. In addition, in attaching each sample slice to a sample plate, it is difficult to attach all the samples to the same position. The difference of their positions makes it difficult to enhance the definition of the mass analysis result image particularly in the depth direction. Additionally, in analyzing a biological sample, a two-dimensional distribution result of mass-to-charge ratios may be first examined and then the position of the following analysis for the depth direction may be determined. In this case, continuous sample slices cannot be prepared in advance, and hence a considerably cumbersome and time-consuming operation will be required.    [Patent Document 1] JP-A 2007-66533    [Patent Document 2] JP-A 2007-157353    [Non-Patent Document 1] Kiyoshi Ogawa and five other authors, “Kenbi Shitsuryo Bunseki Sochi no Kaihatsu,” (“Research and Development of Mass Microscope”) Shimadzu Review, Shimadzu Corporation, Mar. 31, 2006, vol. 62, nos. 3•4, pp. 125-135    [Non-Patent Document 2] Takahiro Harada and eight other authors, “Kenbi Shitsuryo Bunseki Sochi ni yoru Seitai Soshiki Bunseki,” (“Biological Tissue Analysis using Mass Microscope”) Shimadzu Review, Shimadzu Corporation, Apr. 24, 2008, vol. 64, nos. 3•4, pp. 139-146